Nuclear suppression from coherent $J/\psi$ photoproduction at the Large Hadron Collider

Abstract Using the data on coherent J / ψ photoproduction in Pb-Pb ultraperipheral collisions (UPCs) obtained in Runs 1 and 2 at the Large Hadron Collider (LHC), we determined with a good accuracy the nuclear suppression factor of S P b ( x ) in a wide range of the momentum fraction x, 10 − 5 ≤ x ≤ 0.04 . In the small-x region x 10 − 3 , our χ 2 fit favors a flat form of S P b ( x ) ≈ 0.6 with approximately a 5% accuracy for x = 6 × 10 − 4 − 10 − 3 and a 25% error at x = 10 − 4 . At the same time, uncertainties of the fit do not exclude a slow decrease of S P b ( x ) in the small-x limit. At large x, S P b ( x ) is constrained to better than 10% precision up to x = 0.04 and is also consistent with the value of S P b ( x ) at 〈 x 〉 = 0.042 , which we extract from the Fermilab data on the A dependence of the cross section of coherent J / ψ photoproduction on fixed nuclear targets. The resulting uncertainties on S P b ( x ) are small, which indicates the potential of the LHC data on coherent charmonium photoproduction in Pb-Pb UPCs to provide additional constraints on small-x nPDFs. We explicitly demonstrate this using as an example the EPPS16 and nCTEQ16 nuclear parton distribution functions, whose uncertainties decrease severalfold after the Bayesian reweighting of the discussed UPC data.